Rab8a vesicles regulate Wnt ligand delivery and Paneth cell maturation at the intestinal stem cell niche

Communication between stem and niche supporting cells maintains the homeostasis of adult tissues. Wnt signaling is a crucial regulator of the stem cell niche, but the mechanism that governs Wnt ligand delivery in this compartment has not been fully investigated. We identified that Wnt secretion is partly dependent on Rab8a-mediated anterograde transport of Gpr177 (wntless), a Wnt-specific transmembrane transporter. Gpr177 binds to Rab8a, depletion of which compromises Gpr177 traffic, thereby weakening the secretion of multiple Wnts. Analyses of generic Wnt/β-catenin targets in Rab8a knockout mouse intestinal crypts indicate reduced signaling activities; maturation of Paneth cells – a Wnt-dependent cell type – is severely affected. Rab8a knockout crypts show an expansion of Lgr5⁺ and Hopx⁺ cells in vivo. However, in vitro, the knockout enteroids exhibit significantly weakened growth that can be partly restored by exogenous Wnts or Gsk3β inhibitors. Immunogold labeling and surface protein isolation identified decreased plasma membrane localization of Gpr177 in Rab8a knockout Paneth cells and fibroblasts. Upon stimulation by exogenous Wnts, Rab8a-deficient cells show ligand-induced Lrp6 phosphorylation and transcriptional reporter activation. Rab8a thus controls Wnt delivery in producing cells and is crucial for Paneth cell maturation. Our data highlight the profound tissue plasticity that occurs in response to stress induced by depletion of a stem cell niche signal.

Summary: In maturing mouse Paneth cells, Wnt secretion is partly dependent on a Rab8a-mediated anterograde transport of Gpr177. Rab8a is required for Paneth cell maturation.

Borinic acid block copolymers: new building blocks for supramolecular assembly and sensory applications

Borinic acid functional groups were incorporated into block copolymers via RAFT polymerization and their supramolecular assembly and sensor applications were investigated.

Global ablation of the mouse Rab11a gene impairs early embryogenesis and matrix metalloproteinase secretion

Rab11a has been conceived as a prominent regulatory component of the recycling endosome, which acts as a nexus in the endo- and exocytotic networks. The precise in vivo role of Rab11a in mouse embryonic development is unknown. We globally ablated Rab11a and examined the phenotypic and molecular outcomes in Rab11a(null) blastocysts and mouse embryonic fibroblasts. Using multiple trafficking assays and complementation analyses, we determined, among multiple important membrane-associated and soluble cargos, the critical contribution of Rab11a vesicular traffic to the secretion of multiple soluble MMPs. Rab11a(null) embryos were able to properly form normal blastocysts but died at peri-implantation stages. Our data suggest that Rab11a critically controls mouse blastocyst development and soluble matrix metalloproteinase secretion.

CDC42 inhibition suppresses progression of incipient intestinal tumors

Mutations in the APC or β-catenin genes are well-established initiators of colorectal cancer, yet modifiers that facilitate the survival and progression of nascent tumor cells are not well defined. Using genetic and pharmacologic approaches in mouse colorectal cancer and human colorectal cancer xenograft models, we show that incipient intestinal tumor cells activate CDC42, an APC-interacting small GTPase, as a crucial step in malignant progression. In the mouse, Cdc42 ablation attenuated the tumorigenicity of mutant intestinal cells carrying single APC or β-catenin mutations. Similarly, human colorectal cancer with relatively higher levels of CDC42 activity was particularly sensitive to CDC42 blockade. Mechanistic studies suggested that Cdc42 may be activated at different levels, including at the level of transcriptional activation of the stem cell-enriched Rho family exchange factor Arhgef4. Our results indicate that early-stage mutant intestinal epithelial cells must recruit the pleiotropic functions of Cdc42 for malignant progression, suggesting its relevance as a biomarker and therapeutic target for selective colorectal cancer intervention.

TLR sorting by Rab11 endosomes maintains intestinal epithelial-microbial homeostasis

Compartmentalization of Toll-like receptors (TLRs) in intestinal epithelial cells (IECs) regulates distinct immune responses to microbes; however, the specific cellular machinery that controls this mechanism has not been fully identified. Here we provide genetic evidences that the recycling endosomal compartment in enterocytes maintains a homeostatic TLR9 intracellular distribution, supporting mucosal tolerance to normal microbiota. Genetic ablation of a recycling endosome resident small GTPase, Rab11a, a gene adjacent to a Crohn's disease risk locus, in mouse IECs and in Drosophila midgut caused epithelial cell-intrinsic cytokine production, inflammatory bowel phenotype, and early mortality. Unlike wild-type controls, germ-free Rab11a-deficient mouse intestines failed to tolerate the intraluminal stimulation of microbial agonists. Thus, Rab11a endosome controls intestinal host-microbial homeostasis at least partially via sorting TLRs.

Lesion-induced neurogenesis in the hypothalamus is involved in behavioral recovery in adult ring doves

Although neurogenesis in the brain of adult vertebrates is region dependent, lesion induces generation of new neurons in non-neurogenic brain regions. These findings raise the question of the role of new neurons in brain repair and functional recovery. We addressed this question by applying previous observations that electrolytic lesion induced neurogenesis in the ventromedial nucleus (VMN) of the hypothalamus in adult ring doves. Such lesions disrupted the male's courtship behavior, which could be reinstated after rehabilitation with a female. We investigated whether lesion-induced newborn neurons in the VMN facilitate the recovery of courtship behavior in the lesioned birds. We conducted systematic observations of cytological, morphological, and neuroanatomical changes in the lesioned VMN, and concurrently we monitored behavioral changes. Using a multitude of specific cell markers, we found a well-circumscribed cellular zone that proliferated actively. This highly proliferative zone initially appeared along the periphery of the lesion site, where cells had high levels of expression of neuronal, glial, and neurovascular markers. As newborn neurons matured at the lesion site, the necrosis gradually decreased, whereas a downsized proliferative zone relocated to a region ventral to the VMN. Some of the mature neurons were found to project to the midbrain vocal nuclei. Restoration of these projection neurons coincided with the recovery of courtship vocalization. Finally, we found that a social factor, that is, when the male doves were cohoused with a mate, facilitated neurogenesis and behavioral recovery. These results suggest that lesion-induced neurogenesis contributes to behavioral recovery in adult animals.

Seawi--a sea urchin piwi/argonaute family member is a component of MT-RNP complexes

The piwi/argonaute family of proteins is involved in key developmental processes such as stem cell maintenance and axis specification through molecular mechanisms that may involve RNA silencing. Here we report on the cloning and characterization of the sea urchin piwi/argonaute family member seawi. Seawi is a major component of microtubule-ribonucleoprotein (MT-RNP) complexes isolated from two different species of sea urchin, Strongylocentrotus purpuratus and Paracentrotus lividus. Seawi co-isolates with purified ribosomes, cosediments with 80S ribosomes in sucrose density gradients, and binds microtubules. Seawi possesses the RNA binding motif common to piwi family members and binds P. lividus bep4 mRNA, a transcript that co-isolates with MT-RNP complexes and whose translation product has been shown to play a role in patterning the animal-vegetal axis. Indirect immunofluorescence studies localized seawi to the cortex of unfertilized eggs within granule-like particles, the mitotic spindle during cell division, and the small micromeres where its levels were enriched during the early cleavage stage. Lastly, we discuss how seawi, as a piwi/argonaute family member, may play a fundamentally important role in sea urchin animal-vegetal axis formation and stem cell maintenance.

Rho overexpression leads to mitosis-associated detachment of cells from epithelial sheets: a link to the mechanism of cancer dissemination

Dissemination of neoplastic cells from the primary tumor (invasion and metastasis) is a fundamentally dangerous step in multistage carcinogenesis. Recent evidence suggests that Rho GTPase-mediated signaling is linked to dissemination of cells from several different types of human tumors. The Rho family of proteins is typically associated with the regulation of cytoskeletal activity, including actin assembly, microtubule dynamics, and myosin II-dependent contractility of the actin-rich cortex. We examined the effect of overexpression of constitutively active RhoA on islands and monolayers of epithelial cells. Although newly plated cells initially formed small spread islands, there was also a significant population of cells that detached from the substrate, floated in the medium, and then could reattach to the substrate to form new colonies. Detachment of cells from transfected epithelial islands or monolayers occurred in correlation to the plane of cytokinesis after misorientation of the mitotic spindle axis. We suggest that these alterations result from Rho-induced increase of contractility of the cortex of dividing cells, which, during cytokinesis, produces a cell that has budded out of an existing layer of cells. Cell division-mediated detachment of cells from tissue structures may be an important mechanism of tumor dissemination and metastasis.

Functional restoration of acoustic units and adult-generated neurons after hypothalamic lesion

The hypothalamus of the adult ring dove contains acoustic units that respond to species-specific coo vocalization. Loss of nest coo leads to unsuccessful breeding. However, the recovery of nest coo in some doves suggests that these units are capable of self-renewal. We have previously shown that lesioning the hypothalamus generates the addition of new neurons at the lesioned area. In this study, we sought to determine whether lesion-induced new neurons are involved in the recovery of coo-responsive units. We systematically recorded electrical activity in the ventromedial nucleus (VMN) of the hypothalamus, before and after lesion, for varying periods up to 3 months. Recordings were made when the birds were at rest (spontaneous discharge) and when the birds were exposed to acoustic stimulations (evoked discharge). Concurrently, the lesioned area was monitored for changes in cell types by using bromodeoxyuridine (BrdU) to label newly divided cells and NeuN to identify mature neurons. For 1 month after lesion, there was no sign of electrical activity, and only BrdU-labeled cells were present. When the first electrical activity occurred, it displayed abnormal spontaneous bursting patterns. The mature discharge patterns (both spontaneous and evoked) occurred after detection of BrdU+/NeuN+ double-labeled cells 2-3 months postlesion and were similar to those found in intact and sham-lesioned birds. Double-labeled cells bore morphologic characteristics of a neuron and were confirmed with z-stack analysis using confocal laser scanning microscopy. Moreover, double-labeled cells were not stained for glial fibrillary acidic protein (GFAP), suggesting that they were neurons. The number of coo-responsive units was significantly correlated with that of BrdU+/NeuN+ cells. Furthermore, the marker for recording sites revealed that coo-responsive units were colocalized with BrdU+/NeuN+ cells. Taken together, the evidence strongly suggests that lesion-induced addition of new neurons promotes the functional recovery of the adult hypothalamus.

Rho-dependent formation of epithelial "leader" cells during wound healing

The motile behavior of epithelial cells located at the edge of a large wound in a monolayer of cultured cells was analyzed. The initial cellular response is alignment of the edge with an accompanying formation of tangential marginal actin bundles within individual cells positioned along the wound edge. Later, coherent out-growths of cell masses occur by the formation of special "leader" cells at the tops of outgrowths and "follower" cells along the sides. Leader cells exhibit profound cytoskeletal reorganization, including disassembly of marginal bundles, the realignment of actin filament bundles, and penetration of microtubules into highly active lamellae. Additionally, cell-cell contacts acquire radial geometry indicative of increased contractile tension. Interestingly, leader cells acquire a cytoskeletal organization and motility typical of fibroblasts. IAR-2 cultures stably transfected with a dominant-negative mutant of RhoA or treated with Rho-kinase inhibitor Y-27632 transformed most edge cells into leader-like cells. Alternatively, transfection of cells with constitutively active RhoA suppressed formation of leaders. Thus, expansion of the epithelial sheet involves functional differentiation into two distinct types of edge cells. The transition between these two patterns is controlled by Rho activity, which in turn controls the dynamic distribution and activity of actin filament bundles, myosin II, and microtubules.

Mechanisms of polarization of the shape of fibroblasts and epitheliocytes: Separation of the roles of microtubules and Rho-dependent actin-myosin contractility

Cultured fibroblasts possess a characteristic polarized phenotype manifested by an elongate cell body with an anterior lamella whose cell edge is divided into protrusion-forming and inactive zones. Disruption of the fibroblast microtubule cytoskeleton leads to an increase in Rho-dependent acto-myosin contractile activity and concomitant loss of structural polarity. The functional relationship of myosin-driven contractile activity to loss of fibroblast anterior-posterior polarity is unknown. To dissect the roles of microtubule assembly and of Rho-dependent contractility on structural polarization of cells, polarized fibroblasts and nonpolarized epitheliocytes were treated with the microtubule-depolymerizing drug, nocodazole, and/or the Rho kinase inhibitor, Y-27632. Fibroblasts incubated with Y-27632 increased their degree of polarization by developing a highly elongate cell body with multiple narrow processes extended from the edges of the cell. Treatment of fibroblasts with nocodazole, alone or in combination with Rho kinase inhibitor, produced discoid or polygonal cells having broad, flattened lamellae that did not form long lamellar extensions. Single cultured epitheliocytes of the IAR-2 line do not display anterior-posterior polarization. When treated with Y-27632, the cells acquired a polarized, elongate shape with narrow protrusions and wide lamellas. Nocodazole alone or in combination with Y-27632 did not change the discoid shape of epitheliocytes, however treatment with Y-27632 produced thinning of the lamellar cytoplasm. We conclude that microtubules provide the necessary framework for polarization of fibroblasts and epitheliocytes, whereas Rho-regulated contractility modulates the degree of polarization of fibroblasts and completely inhibits polarization in epitheliocytes.

Contact interactions between epitheliocytes and fibroblasts: formation of heterotypic cadherin-containing adhesion sites is accompanied by local cytoskeletal reorganization

Contact interactions between different cell types play a number of important roles in development, for example in cell sorting, tissue organization, and ordered migration of cells. The nature of such heterocellular interactions, in contrast to interactions between cells of the same type, remains largely unknown. In this report, we present experimental data examining the dynamics of heterocellular interactions between epitheliocytes and fibroblasts, which express different cadherin cell adhesion molecules and possess different actin cytoskeletal organizations. Our analysis revealed two striking features of heterocellular contact. First, the active free edge of an epitheliocyte reorganizes its actin cytoskeleton after making contact with a fibroblast. Upon contact with the leading edge of a fibroblast, epitheliocytes disassemble their marginal bundle of actin filaments and reassemble actin filaments into a geometric organization more typical of a fibroblast lamella. Second, epitheliocytes and fibroblasts form cell--cell adhesion structures that have an irregular organization and are associated with components of cell adhesion complexes. The structural organization of these adhesions is more closely related to the type of contacts formed between fibroblasts rather than to those between epitheliocytes. Heterotypic epithelio-fibroblastic contacts, like homotypic contacts between fibroblasts, are transient and do not lead to formation of stable contact interactions. We suggest that heterocellular contact interactions in culture may be regarded as models of how tissue systems consisting of epithelia and mesenchyme interact and become organized in vivo.

Characterization of sea urchin unconventional myosins and analysis of their patterns of expression during early embryogenesis

Early sea urchin development requires a dynamic reorganization of both the actin cytoskeleton and cytoskeletal interactions with cellular membranes. These events may involve the activities of multiple members of the superfamily of myosin motor proteins. Using RT-PCR with degenerate myosin primers, we identified 11 myosin mRNAs expressed in unfertilized eggs and coelomocytes of the sea urchin Strongylocentrotus purpuratus. Seven of these sea urchin myosins belonged to myosin classes Igamma, II, V, VI, VII, IX, and amoeboid-type I, and the remaining four may be from novel classes. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I were either completely or partially cloned and their molecular structures characterized. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I shared a high degree of sequence identity with their respective family members from vertebrates and they retained their class-specific structure and domain organization. Analysis of expression of myosin-V, -VI, -VII, and amoeboid-type-I mRNAs during development revealed that each myosin mRNA displayed a distinct temporal pattern of expression, suggesting that myosins might be involved in specific events of early embryogenesis. Interestingly, the onset of gastrulation appeared to be a pivotal point in modulation of myosin mRNA expression. The presence of multiple myosin mRNAs in eggs and embryos provides insight into the potential involvement of multiple specific motor proteins in the actin-dependent events of embryo development.

Disassembly of actin filaments leads to increased rate and frequency of mitochondrial movement along microtubules

In activated sea urchin coelomocytes, cytoplasmic organelles move along distinct actin and microtubule dependent pathways, actin-based motility is driven by an unconventional myosin, and microtubule disassembly does not effect actin-dependent organelle motility [D'Andrea et al., 1994: J. Cell Sci. 107:2081-2094]. Given the growing evidence for potential interactions between components of the actin and microtubule cytoskeletons, we examined the effect of actin filament disassembly on the movement of mitochondria along microtubules in activated coelomocytes. Coelomocytes treated with cytochalasin B (CB), to disrupt actin filaments, exhibited a thinning of the cytoplasm, enhanced lateral undulation of microtubules, and ceased centripetal cortical flow of actin. Interestingly, the loss of actin filaments resulted in a approximately 1.5-fold increase in the average velocity of outward and inward moving mitochondria and increased the frequency of centripetal movement. To test if enhanced motility along microtubules was a consequence of decreased actin-myosin interaction, coelomocytes were treated with 2,3-butanedione monoxime (BDM), a potent inhibitor of myosin activity [Cramer and Mitchison, 1995: J. Cell Biol. 131:179-189]. BDM inhibited all types of actin-based motility observed in these cells including retrograde cortical flow, protrusion and retraction of the cell edge, and movement of intracellular organelles. Surprisingly, BDM treatment stopped the movement of mitochondria in CB-exposed cells, suggesting that BDM can also act as an inhibitor of organelle movement along microtubules. Collectively, these data demonstrated that microtubule-dependent mitochondrial motility and microtubule movement were sensitive to changes in the assembly state of the actin cytoskeleton.

Myosin-dependent contractile activity of the actin cytoskeleton modulates the spatial organization of cell-cell contacts in cultured epitheliocytes

The spatial organization of cell-cell adherens junctions is distinct in cultured cells from two different tissue types, specifically, epitheliocytes and fibroblasts. In epitheliocytes, contacts are localized tangentially, along contacting cell edges and in association with circumferential actin bundles. Contacts between fibroblasts are radially oriented; that is, they are perpendicular to the overlapping edges of the cells and are associated with straight bundles of actin filaments. In the present study, we establish that the spatial organization of cell-cell contacts in the epithelial cell line IAR-2 can be converted from the typical tangential pattern to the radial pattern observed in fibroblasts. This transition can be induced by treatment with two agents, phorbol 12-myristate 13-acetate and nocodazole, which have different modes of action. Inhibition of myosin contractility reverses tangential-to-radial conversion of cell-cell contacts. These data suggest that formation of radially aligned contacts depends on modulation of contractility within the actin cytoskeleton through the myosin motor protein. The results open the possibility that modulation of the spatial organization of cell-cell contacts may play important roles in regulating organization and physiological functions of epithelial tissues.

Analysis of actin filament bundle dynamics during contact formation in live epithelial cells

The actin cytoskeleton is an integral component of the cell-cell adherens junction complex. We used fluorescence labeling of actin filaments and time-lapse laser scanning confocal microscopy to investigate the functional relationship between the organization of the actin cytoskeleton and formation of adherens junctions in live epithelial cells. Rhodamine-phalloidin was loaded into cultured cells by wounding epithelial monolayers in the presence of fluorescent analog. Rhodamine-phalloidin was incorporated into the actin filaments in stress fibers, circumferential bundles, and marginal bundles. Cells containing labeled actin filaments appeared physiologically normal since the rates of migration, rates of pseudopodial protrusion/retraction, ability to form contacts, and sensitivity to cytochalasin B were equivalent to non-loaded, control epithelial cells. Marginal actin bundles initially formed as bow-shaped bundles that were observed to straighten as the bundles flowed rearward and away from the free cell edge. When lamellae from adjacent cells made contact, rearward flow of marginal bundles ceased and the bundles started to disassemble with higher frequency. Next, we observed the formation of arc-like bundles at the edges of contacting cells, a position suggestive of a role in lateral expansion of the contact. During later stages of contact formation, new actin bundles assembled along the length of the expanding cell-cell boundary. These newly formed bundles are likely to participate in the establishment of the initial cadherin/actin cytoskeleton linkage and eventually form the circumferential bundles at the cell-cell adherens junction. Additionally, indirect immunolocalization studies characterized the location of myosin-II. A model is presented describing the function of the spatial and temporal dynamics of actin filament bundles and myosin-II activity in the formation of adherens junctions.

Dynamics of contacts between lamellae of fibroblasts: essential role of the actin cytoskeleton

We investigated actin cytoskeletal and adhesion molecule dynamics during collisions of leading lamellae of nontransformed and oncogene-transformed fibroblasts. By using real-time video microscopy, it was found that during lamellar collision there was considerable overlapping of leading lamellae followed by subsequent retraction. Overlapping of nontransformed fibroblasts was accompanied by formation of beta-catenin-positive contact structures organized into strands oriented parallel to the long axis of the cell that were associated with bundles of actin filaments. Maintenance of such cell-cell contact structures critically depended on the contractility of actin cytoskeleton, as inhibition of contractility with serum-free medium or 2,3-butanedione 2-monoxime (BDM) resulted in loss of strand formation. Strand formation was recovered when cells in serum-free medium were incubated with the microtubule inhibitor nocodazole, which is known to increase contractility. Oncogene-transformed fibroblasts reacted to collisions with responses similar to nontransformed fibroblasts but did not develop well-organized cell-cell contacts. A model is presented to describe how differences in the organization of the actin cytoskeleton could account for the structurally distinct responses to cell-cell contact by polarized fibroblastic cells versus nonpolarized epithelial cells.